In general, combining beams from multiple lasers overcomes the power limitations of an individual laser. For instance, when combining continuous-wave (CW) laser beams, various methods such as active coherent phasing, spectral combining, passive self-locked combining, and incoherent addition of multiple laser beams have been used. There are various differences between these methods in achieving combined-beam brightness. For example, coherent combining can increase beam brightness, while incoherent combining cannot.
With all known active coherent phasing methods, the beams from the multiple lasers, which are parallel, generate an identical signal, and all parallel output beams are then combined. The total combined power is usually linearly proportional to the total number of combined beams. In particular, the total combined power may not exceed the power of an individual laser in the combined array multiplied the total number of combined beams, assuming each laser provides the same power. Accordingly, in the case of combined continuous-wave signals, the maximum power is limited by the energy/power conservation law.
The same beam combining methods have been applied to overcoming pulse energy limitations of each individual laser. For example, coherent combination of parallel beams of multiple pulsed lasers have been demonstrated. Similar to continuous-wave signal combining, for each of the methods, each pulsed laser produces identical pulsed beams and, therefore, combined pulse energy increases linearly with the number of combined beams. Accordingly, the maximum achievable combined pulse energy may also be limited by the energy extractable from each individual laser multiplied by the number of beams. A combined average power is constrained in a similar manner as power in CW combined systems. Average power being related to the pulse energy Epulse through a pulse repetition rate fr as Paverage=Epulse·fr. Pulse repetition rate in these methods is the same for each individual beam and for the combined output.
A different coherent combining approach for generating pulsed output have been proposed and demonstrated. Specifically, a periodic pulsed signal at the input of the system is decomposed (i.e. spectrally separated) into its constituent CW spectral components using a proper spatially-dispersive device, such as diffraction gratings. Each spectral component is then individually amplified in one of the parallel amplifiers with properly controlling the phase of the CW spectral component. Subsequently, a pulsed periodic signal is reconstituted at the system output by recombining all spectral components again into a single beam using a spatially dispersive device, which can be the same as the one used for spectral separation at the input. This approach is based on the theory that each periodic signal A(t) can be represented as Fourier series decomposition provided in equation (1), where each term in the Fourier series is a CW signal of different optical frequency nωr, each frequency being an n-th harmonic of the repetition frequency ωr=2πfr of a periodic signal, and cn is an amplitude of each constituent CW Fourier-series component.
                              A          ⁡                      (            t            )                          =                                            ∑                              p                =                                  -                  ∞                                            ∞                        ⁢                                          A                0                            ⁡                              (                                  t                  -                                      p                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    T                                                  )                                              =                                    ∑                              n                =                                  -                  ∞                                            ∞                        ⁢                                          c                n                            ⁢                              ⅇ                                  ⅈ                  ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                                      ω                    r                                    ⁢                  t                                                                                        (        1        )            
The spectral amplitude cn is calculated using equation (2), where signal period Tp=1/fr. A0(t) is a temporal shape of each individual pulse in the periodic sequence. Each n-th CW Fourier-series component (characterized by frequency nωr and amplitude cn) is orthogonal to all the others, and all these components constitute a set of normal modes of propagation (frequency modes).
                              c          n                =                              1                          2              ⁢              π                                ⁢                                    ∫                              -                                                      T                    p                                    2                                                                              T                  p                                2                                      ⁢                                                            A                  0                                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                                      -                    ⅈ                                    ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                                      ω                    r                                    ⁢                  t                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (        2        )            
According to the above described method, the generated pulse energy Epulse at the output is determined by the total combined optical power PN-channels, which is proportional to N-times the power Pchannel of each CW signal from each parallel amplifier PN-channels˜N·Pchannel, divided by the repetition rate fr of the pulsed signal: PN-channels/fr=Epulse. In other words, the method is free from pulsed-energy limitations of each parallel amplifier, such as the extractable energy and peak power due, to detrimental nonlinear effects and optical damage.
The main limitation to this method is that the number of parallel beams or channels in such a combined laser array should be equal to the total number Nmodes of frequency modes cn in the periodically-pulsed signal. The total number of modes Nmodes is equal to the ratio between the spectral width Δf of the signal (taken as Fourier transform of A0(t)) and the pulse repetition rate fr (i.e. frequency separation between two adjacent spectral modes cn and cn-1): Nmodes=Δf/fr, or, equivalently, to the ratio between the pulse duration Δtp and pulse repetition period Tp:Nmodes=Δtp/Tp.
In practice, achieving high energies would require unacceptably high number of parallel channels. For example, in order to achieve >1 J per pulse for bandwidth-limited pulse durations shorter than ˜10−9 s at ˜1 kW of total combined power would require more than 106 parallel channels, which may not be practical. This required channel number is much larger than in the identical-channel combining schemes described above.
This section provides background information related to the present disclosure which is not necessarily prior art.